Three orange engines sit in a building on the sprawling grounds of the Back River wastewater treatment plant, producing a steady din. And about 20 percent of the electricity the facility needs.

"It's basically a mini power plant," said Ted Atwood, director of the Baltimore City Energy Office.

This is "microgeneration" — operations that look nothing like the sprawling complexes that most of the country relies on for light, heat and air conditioning. Making electricity on site isn't new, but advocates say interest is growing among significant energy users, driven by cheaper prices for natural gas to fuel systems and reliability worries in the aftermath of extreme weather.

The goal: keep electricity flowing even when nearby power lines go down — and save money to boot.

Baltimore's city government, which runs the Back River facility, plans to expand capacity there and build power-generation systems at other sites.

"We're moving forward aggressively," Atwood said. "I'm looking at one building where the energy costs are about $800,000 a year, and after we put this facility in, we will save about a quarter of a million dollars per year."

What Baltimore wants to build are "combined heat and power" systems, more efficient than conventional power plants because their owners can use both the electricity and heat they produce. The heat can be used to heat water or produce steam, which can in turn be used to power absorption chillers that help cool the air.

President Barack Obama called last August for a 50 percent increase in combined-heat-and-power capacity by 2020, directing federal agencies to assist in the effort.

"Meeting this goal would save energy users $10 billion per year, result in … in new capital investment in manufacturing and other facilities that would create American jobs, and would reduce emissions equivalent to 25 million cars," the White House said at the time.

Baltimore Gas and Electric Co. and two other Maryland utilities now offer incentives for building combined heat and power systems, part of an effort to meet state goals for reducing energy use. BGE, which launched its program last summer, said about a dozen projects have applied for the program in its territory.

In the Baltimore region, five were built in the past 10 years, according to the U.S. Department of Energy.

The new applicants include hospitals, industrial users and local government — Baltimore's planned projects are in the mix.

"Customers can save energy, not through behavior or conservation but through using high-efficiency equipment," said Bill Wolf, manager of energy-efficiency programs for BGE.

Because they're already on site, combined heat and power systems don't have to rely on overhead power lines to deliver what they produce. They can be engineered to disconnect from the larger electrical grid when disruptions hit and keep going.

That makes the systems more cost-effective than generators meant only for emergencies, Wolf said.

"If you're going to put out money for a backup generator, you could put a little bit more in and get a CHP unit," he said.

Upfront costs vary by system size and requirements but can run $2 million to $3 million for a 1-megawatt system, Wolf said. The incentives offered by BGE are designed to offset 30 percent or more of the installation costs.

Wolf said the proposed systems range from 75 kilowatts — enough power for a building the size of a small elementary school — to 2.5 megawatts, the equivalent of 33 of those schools.

Even 2.5 megawatts is small scale for a power plant, though. The nuclear reactors at Calvert Cliffs in Southern Maryland produce more than 1,700 megawatts of electricity.

Though the BGE incentives aren't for homeowners, companies do sell combined heat and power systems meant for individual houses, which really puts the micro in microgeneration. Still, experts warn that big and small users of electricity alike aren't necessarily good candidates for the systems because they need to have a continual use for the heat, too.

Thomas Edison's Pearl Street power station in New York was a 19th-century version of combined heat and power, using the "waste" heat given off in the making of electricity to warm nearby buildings. And some of the systems in Maryland go back more than half a century.

Despite that long history, there aren't many in the state —fewer than 25, the Energy Department said. Users range from military bases to the Johns Hopkins Hospital.

Now their number seems poised to expand. Veolia Energy, which operates energy networks in Baltimore and across the world that supply heating and cooling to customers, is just one of the companies thinking of adding combined heat and power systems in the city. The company said it already is one of the largest operators of such systems worldwide.

"Combined heat and power is a proven technology," said John Gibson, vice president of Veolia Energy North America's South region. "It's been around for a long, long time."

Rob Thornton, president and CEO of the International District Energy Association, expects growth in all sorts of "microgrids" that allow users to operate independently from utilities when necessary.

"What I think is driving this notion of microgrid has been just the frequency and severity of grid interruptions," said Thornton, pointing to last year's derecho and superstorm Sandy as recent examples.

Connecticut launched a microgrid pilot project last year, an idea born of widespread power outages the year before. It's a shift from the long-standing assumption that bigger — and farther away — is better when it comes to power generation.

Cost alone drives some of the interest. Baltimore Mayor Stephanie Rawlings-Blake said in an interview earlier this year that she wants to push energy expenses down so that money can be spent on other priorities.

"I get excited when we talk about ways we can generate more energy ourselves," she said.

The city's nearly 5-year-old combined heat and powersystem at Back River, on Eastern Avenue in Baltimore County, runs on biogas created by the waste digesters on site. That means lower greenhouse gas emissions but also less power generation than the engines could otherwise produce — often 2 megawatts rather than 3 — because the amount of available biogas varies.

It's cost-effective even so, said Atwood of the city energy office. But he's planning to bring the system up to at least 4 megawatts — 40 percent of the facility's average usage— and supplement it with natural gas. He already knows what they'll use the extra heat for.

"There's a big tank of sludge that needs to be heated up before it goes into the digester," Atwood said.

The city is seeking BGE incentives for that project and for other wastewater and water treatment facilities and city buildings.Atwood said microgeneration has interested him for a while, but he got serious about it after the lower cost of natural gas made such projects more cost- effective.

The trio of engines that constitute the city's system at Back River are each about 6 feet wide and 22 feet long, not counting the bevy of pipes snaking out. Atwood is excited about the potential for more power plants this size in a country long accustomed to thinking big.

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